Zinc oxide piezoelectric crystal film

Abstract: Disclosed herein is a zinc oxide piezoelectric crystal film which is grown on an R-plane sapphire substrate to orient its c-axis in parallel with the substrate surface. In order to improve orientation of the zinc oxide piezoelectric crystal film, a Zn target for forming the zinc oxide piezoelectric crystal film by sputtering contains not more than 5 percent by weight of nickel or not more than 4.5 percent by weight of iron with respect to Zn.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a zinc oxide piezoelectric crystal film which is grown on an R-plane sapphire substrate to orient its c-axis generally in parallel with the substrate surface, and more particularly but not exclusively, it relates to an improvement for attaining excellent orientation. Description of the Background Art A typical example of a piezoelectric substrate for a surface wave device comprises a piezoelectric crystal film which is provided on a non-piezoelectric substrate. In relation to a surface wave substrate of such a type, known is a surface wave substrate which is formed by a non-piezoelectric substrate of sapphire (α-Al 2 O 3 ) and a piezoelectric crystal film of zinc oxide (ZnO). Fig. 2 shows a surface wave substrate 3 comprising a ZnO piezoelectric crystal film 2 which is provided on a sapphire substrate 1. In relation to such a surface wave substrate 3, it is known that a ZnO piezoelectric crystal film 2 of a (11 2 0) plane is grown on the sapphire substrate 1 when the same is prepared from (01 1 2) plane cut sapphire, i.e., R-plane sapphire. It is also known that the surface wave substrate 3 provides a high sound velocity and high electromechanical coupling factor when the crystal planes of ZnO and α-Al 2 O 3 are oriented as (11 2 0)ZnO//(01 1 2)α-Al 2 O 3 and crystal axes are regularised as [0001]ZnO//[0 1 11]α-Al 2 O 3 as shown by arrows in Fig. 2, i.e., when the c-axis ([0001]direction) of ZnO is epitaxially grown in parallel with the substrate surface of the sapphire substrate 1. In order to form a ZnO epitaxial film on sapphire, a process such as chemical transportation, CVD or sputtering is employed in general, while sputtering is widely employed among such processes, in particular. When a pure ZnO thin film is formed on an R-plane sapphire substrate by sputtering, for example, its orientation is still insufficient for putting a piezoelectric substrate for a surface wave device which is formed by such a ZnO thin film into practice, although the c-axis in the ZnO thin film is regularized to some extent with respect to the substrate surface, i.e., a certain degree of orientation is attained. For example, the piezoelectric substrate is insufficient in piezoelectricity, and characteristics such as an electromechanical coupling factor are smaller than predetermined values. US-A-4501987 describes a surface acoustic wave transducer having a multilayered substrate which is made of a non-piezoelectric plate which has a surface which is coated with a piezoelectric film. SUMMARY OF THE INVENTION Accordingly, it is desirable to attain improvement in crystal orientation of a zinc oxide piezoelectric crystal film which is epitaxially grown on the aforementioned R-plane sapphire substrate. According to one aspect of the present invention, there is provided a zinc oxide piezoelectric crystal film which is grown on generally R-plane sapphire substrate to orient its c-axis generally in parallel with the substrate surface, and characterized in that the zinc oxide piezoelectric crystal film contains nickel or iron. According to another aspect of the present invention, there is provided a method of forming a crystalline zinc oxide film on a crystalline R-plane sapphire substrate so as to orient the zinc oxide crystal axis generally parallel to the substrate surface, characterised by at the formation of the zinc oxide film so that it incorporates nickel or iron. The aforementioned transition metal is prepared from nickel or iron, for example. When nickel is employed, its content is selected preferably to be not more than 5 percent by weight, more preferably in a range of 0.6 to 4 percent by weight, and most preferably in a range of 1 to 3.1 percent by weight with respect to zinc. When iron is employed, on the other hand, its content is selected preferably to be not more than 4.5 percent by weight, more preferably in a range of 0.6 to 3 percent by weight, and most preferably in a range of 0.9 to 2 percent by weight with respect to zinc. It has been proved that a zinc oxide piezoelectric crystal film containing a transition metal such as nickel or iron exhibits excellent orientation, as clearly understood from the following description of an embodiment. Accordingly, it is possible to put a surface wave substrate which is formed by a zinc oxide piezoelectric crystal film into practice in a surface wave device. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 illustrates changes of diffraction peak half band widths in ZnO thin films with respect to amounts of addition of nickel and iron to Zn targets; Fig. 2 is a perspective view illustrating a surface wave substrate 3; and Fig. 3 is a sectional view illustrating a sputterer 4. DESCRIPTION OF THE EMBODIMENTS As shown in the aforementioned Fig. 2, processes of forming the (11 2 0) plane ZnO epitaxial piezoelectric crystal film 2 on the R-plane sapphire substrate 1 include chemical transportation, CVD and sputtering, while it is possible to obtain a ZnO epitaxial film which is excellent in surface flatness with high quality crystals at a lower temperature by sputtering, in particular. Fig. 3 shows an RF magnetron sputtering equipment or system 4. This sputterer 4 comprises an airtight container 5, which is provided with a gas inlet port 6 and an exhaust port 7. A substrate 8 and a target 9 are arranged in the airtight container 5 in an opposite manner, and a high-frequency (RF) power source 10 applies a high-frequency voltage across an anode (not shown) and a cathode (not shown) which are positioned on upper and lower surfaces of the substrate 8 and the target 9 respectively. A magnet 11 is located under the target 9. In sputtering by the aforementioned sputtering equipment or system 4, the target 9 which is prepared from ZnO ceramic or a Zn metal is struck by particles of argon or the like introduced from the gas inlet port 6, so that zinc oxide particles thereby jumping out adhere onto the substrate 8, or so that zinc particles as jumping out are reacted with gaseous oxygen to form zinc oxide, which in turn adheres onto the substrate 8. In the following experiment, targets 9 were prepared from those of pure Zn metal, Zn metal doped with nickel and Zn metal doped with iron respectively, to form ZnO thin films on substrates 8 of R-plane sapphire by RF magnetron sputtering through the sputtering equipment or system 4 under various sputtering conditions. RF power, a substrate temperature and a gas (Ar:O 2 = 50:50) pressure were varied in relation to the aforementioned sputtering conditions. The as-obtained ZnO thin films were evaluated by an X-ray diffractmeter process, which is adapted to evaluate crystallinity by diffracted waves obtained by irradiating samples with X-rays. In the ZnO epitaxial films obtained by this experiment, peaks were obtained by diffracted waves from crystal planes which were parallel to the substrates, i.e., (11 2 0) planes of ZnO. Orientation properties were improved to implement high quality films having crystal planes regularized further in parallel as the peak strength values were increased and peak widths at positions half the peak heights, i.e., half band widths were reduced. Table 1 shows strength values and half band widths of diffraction peaks of ZnO(11 2 0) planes obtained by employing pure Zn targets and ZnO targets (Zn-Ni targets) doped with 3 percent by weight of Ni with respect to Zn respectively, forming films under various sputtering conditions and analyzing the same by an X-ray diffractmeter process. [Sputtering Condition] 1 2 3 4 5 RF Power (kW) 2.2 2.2 2.2 1.0 1.0 Substrate Temperature (°C) 130 200 250 200 200 Gas Pressure (x10 -3 Torr) 5 5 5 5 7.6 [ZnO (11 2 0) Plane Peak Strength (cps)] Zn Target 3600 4000 4400 1200 4800 Zn-Ni Target 40000 16400 10800 55000 130000 [ZnO (11 2 0) Plane Peak Half Band Width (°)] Zn Target 2.6 1.5 1.3 2.0 1.3 Zn-Ni Target 0.45 0.80 1.00 0.60 0.40 As understood from Table 1, the ZnO thin films prepared from the targets which were doped with nickel had higher diffraction peak strength values and smaller peak half band widths under all sputtering conditions. Thus, these films can be regarded as having superior crystallinity. When such a ZnO/sapphire substrate is employed as a material for a surface wave substrate, the half band width of the diffraction peak is preferably smaller than about 0.8° in practice. In this point, no films having excellent practicality were obtained under all sputtering conditions, when the pure Zn targets were employed. When the targets were doped with nickel, on the other hand, sufficiently practical ZnO epitaxial films were obtained under proper sputtering conditions. Table 2 shows peak strength values and peak half band widths obtained when nickel contents were changed between 0 and 7 percent by weight with respect to zinc under the sputtering conditions exhibiting the smallest diffraction peak half band width, i.e., conditions 5 (RF power: 1 kW, substrate temperature: 200°C and gas pressure: 7.6 by 10 -3 Torr) among those shown in Table 1. Ni Content (wt %) ZnO (11 2 0 Plane Peak Strength (cps) ZnO (11 2 0) Plane Peak Half Band Width (° ) 0 4800 1.3 1 96000 0.45 3 130000 0.40 5 9200 1.15 7 3800 2.1 Fig. 1 shows relations between the peak half band widths shown in Table 2 and the Ni contents. It is understood from Table 2 and Fig. 1 that crystallinity was improved as compared with the case of employing the Zn targets containing no nickel when the nickel contents were not more than 5 percent by weight. When nickel was added by 7 percent by weight, a diffraction peak from a (0001) plane was also slightly observed in addition to that from a (11 2 0) plane in diffracted waves from the ZnO thin film, which was not a complete epitaxial film. Films prepared from Zn targets which were doped with iron were also examined. Tables 3 and 4 show the results. Table 3 and 4 correspond to the above Tables 1 and 2 respectively. [Sputtering Condition] 1 2 3 4 5 RF Power (kW) 2.2 2.2 1.5 1.0 1.0 Substrate Temperature (°C) 130 200 200 200 200 Gas Pressure (x10 -3 Torr) 5 5 5 5 7.6 Zno (11 2 0) Plane Peak Strength (cps) 42000 14600 22000 34800 74500 ZnO (11 2 0) Plane Peak Half Band Width (°C) 0.60 0.70 0.65 0.60 0.55 Fe Content (wt %) ZnO (11 2 0) Plane Peak Strength (cps) ZnO (11 2 0) Plane Peak Half Band Width (° ) 0 4800 1.30 1 89600 0.50 2 74500 0.55 4 6100 1.00 6 4300 1.75 Fig. 1 also shows relations between the peak half band widths shown in Table 4 and the Fe contents. From Tables 3 and 4 and Fig. 1, it is possible to confirm that the as-obtained ZnO thin films were also improved in crystallinity when the Zn targets doped with iron were employed. As understood from Table 4 and Fig. 1 in particular, the films prepared from the Zn targets which were doped with iron were improved in crystallinity as compared with those prepared from the Zn targets which were doped with no iron, when the Fe contents were not more than 4.5 percent by weight with respect to zinc. Also when the Zn target containing 6 percent by weight of iron was employed, a diffraction peak from a (0001) plane was also slightly observed in addition to that from a (11 2 0) plane in the as-obtained ZiO thin film, which was not a complete epitaxial film, similarly to the aforementioned film which was prepared from the target containing 7 percent by weight of nickel. The content of nickel or iron is shown as a value in the Zn target in the above description, while the content in the target can be regarded as the content of nickel or iron in the as-obtained ZnO thin film with respect to Zn since compositions of the target and the as-obtained film substantially match with each other in sputtering. Although the transition metal is prepared from nickel or iron in the aforementioned embodiment, still another transition metal such as cobalt, for example, can also be advantageously employed. The "R-plane" of sapphire employed as a substrate in the present invention is a (01 1 2) plane, while dispersion of about ±2 degrees appears with respect to the R-plane in practice. However, the effect substantially remains the same within such a range of dispersion. Even if a cut plane of sapphire is displaced by about ±2 degrees, therefore, it is possible to obtain a ZnO epitaxial film of a (01 1 2) plane. Namely, the term "R-plane" employed in this specification is intended to cover such dispersion. It should be noted that the present invention is not limited to the embodiments as described above. It is envisaged that various modifications and variations to the above described embodiments could be made without falling outside the scope of the present invention as determined from the claims.

Claims: A zinc oxide piezoelectric crystal film (2) being grown on a generally R-plane sapphire substrate (1) to orient its c-axis generally in parallel with a surface of said substrate (1), characterised in that said zinc oxide piezoelectric crystal film (2) contains nickel or iron. A zinc oxide piezoelectric crystal film (2) in accordance with claim 1, wherein the content of said nickel is selected to be not more than 5 percent by weight with respect to zinc. A zinc oxide piezoelectric crystal film (2) in accordance with claim 1, wherein the content of said nickel is selected in a range of 0.6 to 4 percent by weight with respect to zinc. A zinc oxide piezoelectric crystal film (2) in accordance with claim 1, wherein the content of said nickel is selected in a range of 1 to 3.1 percent by weight with respect to zinc. A zinc oxide piezoelectric crystal film (2) in accordance with claim 1, wherein the content of said iron is selected to be not more than 4.5 percent by weight with respect to zinc. A zinc oxide piezoelectric crystal film 92) in accordance with claim 1, wherein the content of said iron is selected in a range of 0.6 to 3 percent by weight with respect to zinc. A zinc oxide piezoelectric crystal film (2) in accordance with claim 1, wherein the content of said iron is selected in a range of 0.9 to 2 percent by weight with respect to zinc. A method of forming a crystalline zinc oxide film (2) on a crystalline R-plane sapphire substrate (1) so as to orient the zinc oxide crystal axis generally parallel to the substrate surface, characterised by the formation of the zinc oxide film (2) so that it incorporates nickel or iron.

Inventors: IEKI HIDEHARU, KOIKE JUN, IEKI, HIDEHARU, KOIKE, JUN, IEKI, HIDEHARU, C/O MURATA MANUFACTURING CO., LTD., KOIKE, JUN, C/O MURATA MANUFACTURING CO., LTD.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1994
Main classification: H03H9
IPC codes: H01L41, H03H9, C30B29